Optical imaging systems have many practical applications, including the inspection of material surfaces for defects or specific contours. These particular types of optical imaging systems typically include a source of illumination which can be reflected off the surface being inspected, an imaging lens for focusing the reflected illumination, and an optical detector for sensing this focused pattern of reflected illumination and outputting an analog signal corresponding thereto. The analog signal may then be digitized for further processing and analysis.
The source of illumination is typically a device emitting white light which is applied to the surface being inspected at a fixed angle. The optical detector is positioned relative to the surface at the same relative light receiving angle, and the surface is moved relative to these fixed positions of the illumination source and the optical detector. The light reflected from the portion of the surface upon which light is shined is focused by the imaging lens onto sensing elements in the optical detector.
The optical detector has as its primary function the detection of optical radiation. Detection, in general terms, is the measurement of optical radiation. More specifically, it is the conversion of optical energy to a measurable parameter. Typically this parameter is an electrical quantity such as a voltage or current, represented by an analog signal outputted by the detector. Ideally, detectors exhibit a linear transfer characteristic over a wide dynamic range. In practice, however, the lower end of the useful dynamic range is often limited by noise and the upper end is limited by saturation.
Typically, video cameras are used as the optical detectors in optical imaging surface inspection systems. Sensing elements in the camera detect light reflected from the surface being inspected and output an analog video signal related to the amount of light detected. The signal comprises a series of pulses, each of which represents a portion of sensed reflected light. The white and black levels of this signal correspond to the portions of the signal representative of the reflective and nonreflective areas of the surface, respectively.
Theoretically, the video camera would respond instantaneously to abrupt changes in surface reflected illumination and output a video signal representing a step function. In practice, however, the analog signal generated by the camera representing the surface reflected illumination is sloped due to the response time of amplifiers in the camera, the finite resolution of the sensing elements in the camera, and the grey scale effect of illumination reflected off the surface. Thus in order to produce a signal which accurately represents the particular pattern on the surface being inspected, a threshold level must be selected at which level the analog video signal can be digitized. Typically, a threshold level is selected to obtain the most accurate digital representation of the physical characteristics of the surface. This threshold level is then applied to the entire line of pulses which make up the analog video signal.
The application of a constant threshold level to digitize all the pulses in an analog video signal, however, has several drawbacks. First, the amplitude of each of the pulses in the analog video signal, is increased by background reflected light near the particular portion of the surface at which the primary reflected light is sensed. Thus, the analog signal is positively offset by this background reflected light, the amount of offset referred to as the black level. This background reflected light or black level may be removed from the digitized video signal by offsetting the threshold level by the amount of background reflected light. The black level, however, varies across the surface being inspected and may, in fact, vary for each individual pulse in the analog video signal, resulting in pulses which do not accurately represent the light reflected by the image pattern on the surface being inspected. Applying a constant threshold level to a series of these pulses in the analog video signal, then, will not compensate for a black level which varies over real time.
Second, the level of white light reflected by similarly contoured portions of the surface being inspected may not be the same. This varied degree of reflectance may by caused by variances in the output of the illumination source or by variances in the reflectivity of similarly contoured portions of the pattern. Weak video signal pulses having amplitudes which fall below the selected threshold level may not be recognized or subsequently digitized. Application of a constant threshold level to the video pulse train will thereby result in an inaccurate digital representation of the surface being inspected.
Third, the video camera may not output the same analog signal for a given level of illumination. A low video signal level may result from low sensitivity of the video camera sensing elements or low output from the video camera charge amplifiers. Thus, this non-uniformity of the video camera components may result in pulses in the analog signal having varied peak amplitudes for similar portions of the surface being inspected. A constant threshold level applied to a series of these pulses will not compensate for this peak amplitude variance.
Nonuniform digital imaging may result from any one of the above problems, and a cumulative adverse effect on the digital image may result if more than one of these problems is present. If the adverse effect caused by each of these problems could be adequately characterized, a programmed, variable threshold could be applied to the series of pulses in the analog video signal. However, because the characterized effects are likely to change over time, the programmed threshold may not provide the appropriate correction factor, and may in fact further distort the resultant digital image.
Thus, there is a need for an optical imaging surface inspection system which can produce an accurate representation of the image on the surface by utilizing a dynamic thresholding technique which compensates for real time variances in background reflected illumination, reflectance across the surface, and video camera functional capabilities. The present invention addresses this need.